1. Field of the Invention
The present invention relates to a polymer dispersed liquid crystal complex film, a liquid crystal display device, and a method for producing the same. A polymer dispersed liquid crystal display device according to the present invention is, owing to a broad range of viewing angles thereof, applicable to a flat display device such as a liquid crystal panel to be used in an optical projector, a personal computer, an amusement device, or a television set. Moreover, the polymer dispersed liquid crystal display device is, owing to the excellently uniform contrast thereof, applicable to a display device to be used in a projection display. Furthermore, the polymer dispersed liquid crystal display device is, in view of a shutter effect thereof, applicable to a display board, a window, a door, a wall, and the like.
2. Description of the Related Art
There have been realized display devices of a TN (Twisted Nematic) mode or an STN (Super Twisted Nematic) mode, as display devices utilizing electrooptical effects. Both display modes use nematic liquid crystal. A display mode using ferroelectric liquid crystal has also been proposed. However, these display modes require a polarizing plate and an orientation treatment. Display modes which utilize the light-scattering property of liquid crystal but do not require a polarizing plate are, for example, a DS (Dynamic Scattering) mode and a PC (Phase Change) mode.
There has been developed a display device which requires neither a polarizing plate nor an orientation treatment. It is known that such a display device can be realized by dispersing liquid crystal droplets sealed in capsules in a polymer medium so as to form a film. Proposed materials for forming such capsules are gelatine, gum arabic, polyvinylalcohol, and the like, as is disclosed in Japanese National Publication No. 58-501631, U.S. Pat. No. 4,435,047, etc.
This type of conventional art amounts to the following technique: An ordinary refractive index n.sub.o of a liquid crystal molecule and a refractive index n.sub.p of the polymer medium are made to be equal. As a result, a transparent state is displayed when a voltage is applied, since the liquid crystal molecules are oriented in the same direction. A white state is displayed when no voltage is applied, since the liquid crystal molecules are oriented in various directions (random orientation) so that the refractive indices of the liquid crystal droplets deviate from the ordinary refractive index no, causing scattering of light at interfaces of the liquid crystal and the polymer. Inevitably, it is necessary to carefully choose a liquid crystal material and a polymer material in order to ensure that the ordinary refractive index n.sub.o of the liquid crystal and the refractive index n.sub.p of the polymer are approximately the same so that light is sufficiently transmitted when a voltage is applied. Japanese Laid-Open Patent Publication Nos. 63-43993 and 3-245120 disclose methods for producing a transparent liquid crystal panel by ensuring that the ratio of the ordinary refractive index n.sub.o of liquid crystal to the refractive index n.sub.p of a polymer film are in the range of 0.98 to 1.02.
A polymer dispersed liquid crystal display device has also been proposed which utilizes the same principle as that described above. This display device is a liquid crystal display device using a polymer dispersed liquid crystal complex film. The complex film has a structure in which liquid crystal regions, each constituted essentially by a liquid crystal droplet or continuous liquid crystal droplets, are partitioned by polymer walls. A known method for producing such a polymer dispersed liquid crystal display device is disclosed in Japanese National Publication No. 61-502128. In this method, in order to attain sufficient control of the diameters of liquid crystal droplets, the above-mentioned liquid crystal display device is produced through phase separation, starting from a mixture of a liquid crystal material and a light-curable (photopolymerizable) material.
Some conventional modes in which scattering of light occurring at interfaces of liquid crystal regions and polymer walls is controlled are applicable to a polymer dispersed liquid crystal device. Examples of such conventional modes are a TN mode, an STN mode, an ECB (electrically controlled birefringence) mode, and an FLC (Ferroelectric Liquid Crystal) mode.
In a case where a polymer dispersed liquid crystal device is used as a liquid crystal device for optical projection, or as a transmission liquid crystal display device provided with a back light, display characteristics of the liquid crystal device are required to be stable against heat for long periods so as to cope with heating effects due to emission of light by a metal halide lamp used as a light source, a cold-cathode tube, an EL film, and the like, and/or emission of heat in the operation of the device.
However, a liquid crystal display device produced with the above method has the following problems: a) a display panel of the liquid crystal display device has poor resistance against heat because of a mutual dissolution effect of the liquid crystal material and the polymer (light-curable) material, that is, dissolution of unreacted light-curable monomers and oligomers into the liquid crystal material and elution of the liquid crystal material into the polymer material; b) display characteristics of the liquid crystal panel are influenced by changes in thermal circumstances; c) the response speed of the liquid crystal decreases because the viscosity of the liquid crystal material increases owing to the above-mentioned mutual dissolution effect.
Japanese Laid-Open Patent Publication Nos. 3-219211, 4-1724, 4-70714, etc. are known to deal with improvement of heat resistance of liquid crystal display devices, describing a glass transition temperature T.sub.g of a polymer material to be used for a matrix. However, in considering thermal characteristics of a whole panel of a polymer dispersed liquid crystal device or a polymer-matrix type liquid crystal device, influence of the liquid crystal material, which is a main component of the device, must be taken into account. In particular, thermal characteristics of a complex system consisting of materials of different kinds, such as a liquid crystal-polymer complex film, are known to be different from those of a single material film. Therefore, it is insufficient to pay attention only To the glass transition temperature T.sub.g of the polymer material To consider heat resistance that affects the display characteristics. Moreover, any of the methods for producing a polymer dispersed liquid crystal device disclosed in the above publications is a `solvent evaporation method`, in which the liquid crystal material and the polymer material are dissolved in a common solvent, applied to a substrate, and then dried so as to form a thin film containing liquid crystal. Since it is difficult to control a phase separation process of a mixture of the liquid crystal material and the polymer material by this solvent evaporation method, a polymer dispersed liquid crystal device produced thereby inevitably has a very high driving voltage. As a result, such a polymer dispersed liquid crystal, when applied to a flat panel display of an active matrix driving system, has a problem that the flat panel display is difficult to be realized since the driving voltage therefor exceeds the withstand voltage of an LSI circuit used in a driving circuit.
Effects of a glass transition temperature T.sub.g of a polymer material on electrooptical characteristics of polymer dispersed liquid crystal are also described in "Appl. Phys. Lett. 60, 3238 (1992)". However, a liquid crystal device disclosed in this article is also produced by a solvent evaporation method, and therefore has a major problem that its driving voltage exceeds 50 V.
Moreover, "Extended Abstracts", p. 310 (The 17th Liquid Crystal Forum, 1991) reports on a relationship between voltages and light transmittances, and on methods to realize a higher response speed, with respect to a polymer dispersed liquid crystal display device in which small amounts of cholesteric liquid crystal and smectic liquid crystal are added. However, the liquid crystal display device reported in the above has such display quality problems that contrast thereof is low because of insufficient light-scattering characteristics, and that the voltage-transmittance hysteresis is large, and that it is difficult to realize intermediate gray tones.
On the other hand, Japanese Laid-Open Patent Publication No. 4-14015 discloses a material including a polymer containing fluorine, as a resin material for forming a polymer wall. However, no attention is paid to influences of thermal circumstances on display characteristics, including heat resistance, of a liquid crystal panel in the structure. The structure therefore has poor practicality.
A conventional liquid crystal display mode which utilizes optical rotatory power of liquid crystal molecules is known to have a major problem of insufficient viewing angle characteristics. This problem will be described with reference to FIGS. 16A to 16C, which are cross-sectional views showing a conventional liquid crystal display device of a TN mode. As is shown in FIGS. 16A to 16C, this liquid crystal display device of a TN mode has a structure in which a display medium including liquid crystal molecules 11 are interposed between a pair of substrates 12 and 13. FIG. 16A describes an initial state of the liquid crystal display device; FIG. 16B describes a state where the liquid crystal display device is displaying an intermediate gray tone; FIG. 16C describes a state where the liquid crystal device is being transparent.
In an initial state, as is shown in FIG. 16A, the liquid crystal molecules 11 are twisted by 90.degree., and also stand at a certain angle (pre-tilt angle) in one direction. In a transparent state, as is shown in FIG. 16C, the liquid crystal molecules 11 are oriented vertically with respect to the substrates 12 and 13. Therefore, each liquid crystal molecule 11 stands at the same angle, irrespective of the viewing angle. In other words, each liquid crystal molecule 11 has the same apparent refractive index irrespective of the viewing angle. However, in a state where the liquid crystal display device is displaying an intermediate gray tone, as is shown in FIG. 16B, since the liquid crystal molecules 11 are oriented at certain angles with respect to the normal line of the transparent substrates 12 and 13, the liquid crystal molecules 11, as observed from a direction A, stand at different angles than in cases where they are observed from a direction B. In other words, the apparent refractive index of each liquid crystal molecule 11 varies depending on the viewing angle. As a result, contrast of an image displayed on the display device greatly varies depending on whether the image is observed from the direction A or the direction B. In some extreme cases, display defects such as inversion of the image occur. As is described above, conventional display modes have a problem of inadequate viewing angle characteristics.
The inventors previously invented a polymer-matrix type liquid crystal display device in which a liquid crystal material is gathered in pixel regions of a liquid crystal panel, and liquid crystal domains are disposed radially in each pixel region. The liquid crystal display device is bright, and has drastically improved viewing angle characteristics. The liquid crystal display device, in reference to the structure thereof, includes a pair of substrates, polymer walls (which consist essentially of a polymer material) arranged in a matrix shape interposed between the substrates, liquid crystal regions partitioned by the polymer walls, and at least one polarizing plate formed on the substrates. Within each liquid crystal region, liquid crystal molecules gradually stand in the directions of the polymer walls as a voltage is applied, owing to an interaction of the liquid crystal molecules and the polymer walls. Therefore, as is described above, the apparent refractive index of the liquid crystal molecules as a whole becomes substantially the same irrespective of the viewing angle, whereby the viewing angle characteristics are greatly improved.
The polymer matrix type liquid crystal display device uses a mixture consisting of materials similar to those which were disclosed in the above-mentioned Japanese National Publication No. 61-502128, and is produced by irradiating the mixture with light, while disposing a light-intercepting object such as a photomask over the pixel regions. Accordingly, this liquid crystal display device, invented by the inventors, has the same problems as those of the conventional polymer dispersed liquid crystal display device disclosed in Japanese National Publication No. 61-502128.
Moreover, in the above-mentioned liquid crystal display device, liquid crystal domains of each liquid crystal region are oriented either radially or randomly with respect to the center of the corresponding pixel. Therefore, scattering of light occurs locally at interfaces of the liquid crystal and the polymer walls in cases where the liquid crystal and the polymer walls have a large difference in refractive indices thereof. Accordingly, there has also been a problem of low display quality, etc. because the contrast of the display device is low and a displayed image has a coarse appearance.